The paper contains a survey of results about the possibility of inscribing convex polygons of particular types into a plane convex figure. It is proved that if K is a smooth convex figure, then K is circumscribed either about four different reflection-symmetric, convex, equilateral pentagons or about a regular pentagon.Let S be a family of convex hexagons whose vertices are the vertices of two negatively homothetic equilateral triangles with common center. It is proved that if K is a smooth convex figure, then K is circumscribed either about a hexagon in S or about two pentagons with vertices at the vertices of two hexagons in S. In the latter case, the sixth vertex of one of the hexagons lies outside K, while the sixth vertex of another one lies inside K. Bibliography: 15 titles.By a convex figure below we mean a compact convex subset K of the plane with nonempty interior. We say that a convex polygon P is inscribed in K (circumscribed about K) if the vertices of P lie on the boundary of K (the sides of P are lines of support for K). In these cases, we also say that K is circumscribed about P (inscribed in P ).The square is the first one of the quadrangles about which it was proved that they can be inscribed in any convex figure. The history of this theorem is described in [4]. (Somewhat later, Shnirel'man [1] proved that the square can be inscribed in any C 2 -smooth Jordan curve on the plane.) The present author has repeatedly proposed a conjecture that each regular Jordan curve is circumscribed about a similar image of any quadrangle inscribed in a circle. This assertion is proved in [9] only for ovals with four vertices. It is also proved that each star-shaped regular Jordan curve contains four vertices of a regular pentagon.Many authors proved that each strictly convex figure is circumscribed about a positively homothetic image of a prescribed triangle. (A more general assertion in all dimensions is proved in [11].) It is proved in [6, 7] that each convex figure is circumscribed about an affine image of a regular pentagon (an affine-regular pentagon) with vertex at a prescribed boundary point of the figure. The same is proved in [8] for an arbitrary convex pentagon P such that the sum of any two neighboring angles of P is greater than π.It is proved in [2] that each convex figure is circumscribed about an affine image of a regular hexagon, and the same is proved in [13,14] for an arbitrary centrally symmetric convex hexagon.It is proved in [7] that each centrally symmetric convex figure is circumscribed about an affine image of the regular octagon, and the same is proved in [12] for the centrally symmetric octagon with vertices at the vertices of the regular decagon.
Theorem 1. Each smooth convex figure K is circumscribed either about four distinct mirrorsymmetric equilateral convex pentagons or about a regular pentagon.Proof. We consider the product (∂K) 5 of five copies of the oriented boundary ∂K of K and the open subset M ⊂ (∂K) 5 consisting of quintuples of distinct boundary points of K going one after...